This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved apparatus for controlling the lateral alignment of a moving photoconductive belt.
In an electrophotographic printing machine, a photoconductive belt is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive belt is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive belt selectively discharges the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive belt corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive belt, the latent image is developed by bringing a developer mix into contact therewith. Generally, the developer mix comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive belt. The toner powder image is then transferred from the photoconductive surface to a copy sheet. Finally, the copy sheet is heated to permanently affix the toner particles thereto in image configuration. This general approach was originally disclosed by Carlson in U.S. Pat. No. 2,297,691, and has been further amplified and described by many related patents in the art.
It is evident that the lateral alignment of the photoconductive belt is critical and must be controlled within prescribed tolerances. This is due to the fact that the photoconductive belt passes through many processing stations during the printing operation. As the belt passes through each of these processing stations, the location of the latent image must be precisely defined in order to optimize each operation being performed thereon. If the position of the latent image deviates from processing station to processing station, copy quality will be significantly degradated. Thus, lateral movement of the photoconductive belt must be minimized so that the belt moves in a pre-determined path.
Ideally, if the photoconductive belt was perfectly constructed and entrained about perfectly cylindrical rollers mounted and secured in an exactly parallel relationship with one another, the velocity vector of the belt would be substantially normal to the longitudinal axis of the roller and there would be no lateral walking of the belt. However, in actual practice, this is not feasible. Frequently, the velocity vector of the belt approaches the longitudinal axis or axis of rotation of the roller at an angle. This produces lateral movement of the belt relative to the roller. Alternatively, the axis of rotation of the roller may be tilted relative to the velocity vector of the belt. Under these circumstances, the belt will also move laterally. Thus, the photoconductive belt must be tracked or controlled to regulate its lateral position. Existing methods of controlling the lateral movement of the photoconductive belt comprise various forms of crowned rollers, flanged rollers, and electrical servo systems. However, systems of this type may produce high local stresses resulting in damage to the highly sensitive photoconductive belt. Steering rollers employing servo systems to maintain control generally apply less stress on the belt. However, systems of this type are costly.
Accordingly, it is a primary object of the present invention to improve the system controlling the lateral movement of the photoconductive belt employed in an electrophotographic printing machine.